coastal B.C. and part of Alaska probably originated near the northern end of the Caledonian mountain belt, between the continents of Laurentia, Siberia and Baltica.
MAP 4. EARLY DEVONIAN (395 million years ago). The westward travel of the Arctic terranes towards Panthalassa (the precursor to the Pacific Ocean) is believed to have been propelled by a Caribbean-style subduction zone. This subduction zone, its island arcs and continental fragments travelled rapidly along a Paleozoic Northwest Passage.
MAP 5. LATE DEVONIAN–MISSISSIPPIAN (360 million years ago). The northward shift of Euramerica (the now-combined Laurentia and Baltica), during its collision with Gondwana, results in the formation of a subduction zone along the west coast. This subduction begins from the small Caribbean-type zone and moves some of the Arctic terranes southward. The hot upwelling beneath the subduction zone causes a rift in Laurentia, giving birth to the first of the peri-Laurentian arc terranes along the west coast, including the Yukon-Tanana terrane (blue-green).
MAP 6. PENNSYLVANIAN–EARLY PERMIAN (300 to 285 million years ago). Westward retreat of the subduction zone leads to widespread island arc volcanism (green) that develops on fragments of western Laurentia (blue-green) and on some of the Arctic terranes (yellow) that had recently arrived. These new arc terranes include the early expressions of Stikinia and Quesnellia. The Slide Mountain Ocean develops between the island arc and the continental margin in the wake of the westward migration of the arc, mirroring what is happening in the present-day Sea of Japan. The Insular terranes of coastal B.C. (Alexander and Wrangellia) come together far out in Panthalassa. Alexander is one of the Arctic terranes with northern European origins.
The result was our mountains—low ones at first, with the initial collisions, but as the continent continued its inexorable course, sedimentary strata at its margin piled up like snow in front of a vast, majestic snowplow, riding up and over eastward to make the shingled stack that later would be sculpted into the modern Rockies. The physiography of British Columbia—its twin backbones of the Coast Mountains and the Ominecas and Rockies separated by the more subdued Intermontane belt—is the result of the two, slow-motion, simultaneous collisions. Where the Intermontane terranes piled up onto the old continental margin, the Omineca and Rocky Mountains rose. Where the Insular terranes collided with the outer edge of the Intermontane terranes, the Coast Mountains were born.
MAP 7. LATE PERMIAN–EARLY TRIASSIC (250 million years ago). All major continental masses have converged to form the supercontinent Pangaea. Along its west coast, subduction has reversed to consume the Slide Mountain Ocean, returning the peri-Laurentian arc terranes (green) to near the continental margin. Later in Triassic time, subduction flips once more (dashed grey line), and arc volcanism flourishes again on Quesnellia and Stikinia. Alexander and Wrangellia remain at large in Panthalassa.
MAP 8. EARLY JURASSIC (190 million years ago). The Atlantic Ocean is born, growing in the rift between North America, Africa and part of Europe, and propelling North America westward. Buckling of the peri-Laurentian (Intermontane) terranes traps part of the ancient Pacific Ocean floor that was brought to North America by the subduction conveyor belt from far reaches—the Cache Creek terrane. At the same time, the westward-moving North America is on a collision course with the Insular terranes lying offshore in the Pacific. The ultimate collision will build the mountains of western North America and shape the final terrane patchwork (Map 1, page 10).
The Omineca–Rocky Mountain Collision Zone
As North America drove under its western neighbours during the Middle Jurassic, large pieces of Quesnellia and the Slide Mountain terrane began to peel off the oceanic plate. Some slices up to 25 kilometres thick overrode the continental margin, becoming stacked like pancakes on top of it. This stacking makes it difficult to say precisely where the old edge of North America lies today. The rocks of the terranes and the old continental shelf were squeezed and folded to form the Columbia, Omineca and Cassiar Mountains. In some areas the intense compression and consequent heating recrystallized the rocks into the metamorphic rocks of the Omineca and Monashee Mountains and the Quesnel and Shuswap Highlands. Partial melting in some regions gave rise to local intrusive igneous rocks.
A river of golden volcanic rock flows down a steep slope in the Ilgachuz Mountains, north of Anahim Lake.
Compression continued, and the thick layers of sedimentary rocks covering the continental core were pushed ever eastward in front of the colliding wedge and were squeezed, folded and telescoped (Figure 2). The sedimentary layers first were deformed into waves like those in a carpet being pushed. But the strong, resistant limestone layers broke when folded and became stacked up one on top of another in gently sloping piles. These breaks are called thrust faults, and the blocks of rocks above the break are called thrust sheets. By 120 million years ago, the western ranges of the Rockies were stacking up. A deep depression, the Rocky Mountain Trough (not Trench) formed east of the mountain-building wave, the result of the tremendous weight building up on the edge of the continent. The rapid uplift caused massive erosion of the new mountains, and sediments soon piled up in the trough’s inland sea, forming thick deposits of mudstone and shale.
The mountain-building wave in the Rockies continued to move eastward. The main ranges were rising about 100 million years ago and, by the time the pushing stopped about 60 million years ago, the eastern ranges and foothills had been created. When all was said and done, the thrust sheets (Figure 2) had been telescoped and shoved up to 250 kilometres eastward from their original position—the rocks of Mount Rundle, at Banff, were originally laid down somewhere around Revelstoke. As the thrust sheets moved to the east and stacked on top of one another, the Rocky Mountain Trough moved eastward ahead of them. But the thrust sheets overtook the shales that had been deposited in the trough’s earlier position, and layers of these soft shales were caught between the sheets. The shales erode much more easily than the resistant limestones, and this difference results in a pattern that is seen over and over again in the Rockies—hard, limestone cliffs towering over soft, shale-bottomed valleys (Figure 2, page 34).
FIGURE 2: THE FORMATION OF THRUST FAULTS AND THRUST-FAULTED MOUNTAINS. Stage 1: Compression from the left bends and finally breaks the rock layers. Stage 2: The upper sheet of rocks, the “thrust sheet,” is pushed over the lower sheet. Stage 3: The face of the mountain after erosion. Some of the ancient limestones at the bottom of the sedimentary pile (e.g., layer D) end up on top of the younger shales (e.g., layer B). Adapted from C.J. Yorath, Where Terranes Collide, p. 9.
Treadmill Ridge, looking south along the Continental Divide between Jasper National Park and Mount Robson Provincial Park. These gently sloping mountains end in abrupt cliffs to the east, which mark today’s eroded edge of a thrust sheet.
The Coast Mountains Collision Zone
As the Insular terranes ran into the Intermontane
